1. Field of the Invention
The present invention relates in general to a method of determining or detecting the concentration of a component in a gas to be measured, by using a solid electrolyte, and to a electrochemical device suitable for practicing the method. More particularly, the invention is concerned with such method and device for determining the concentration of a given component in the gas to be measured, by utilizing two electrochemical cells as a pumping and sensing cell, each cell comprising a solid electrolyte body and a pair of porous electrodes.
2. Related Art Statement
There are known various types of electrochemical devices or elements using a solid electrolyte material, for example, as A/F ratio sensors for determining the concentration of oxygen or unburned components in exhaust gases produced by an internal combustion engine or a motor vehicle. Such an A/F ratio sensor, which detects an A/F (air/fuel) ratio of an air-fuel mixture supplied to the engine, comprises an electrochemical cell which employs, for example, zirconia ceramics as an oxygen-ion conductive solid electrolyte, and a pair of porous electrodes. The electrochemical cell is adapted to perform a well-known electrochemical pumping action by electrode reaction due to an electric current applied between the pair of electrodes. Meanwhile, one of the electrodes is exposed to the gas (exhaust gases) in an external space, via suitable diffusion-resistance means as in the form of a thin flat space or clearance or a porous ceramic layer, which has a predetermined diffusion resistance to the molecules of the component to be measured. The pumping current flowing through the electrodes represents the concentration of the component such as oxygen or unburned components in the external gas, whereby the concentration is determined. Also known in the art are detectors (electrochemical devices) which are adapted to sense, like the A/F sensors indicated above, various other substances such as water, hydrogen and carbon dioxides.
In such a method of determining the concentration of a given component in a gas, through measurement of an amount of diffusion of the component which varies with the concentration of the component, by using an electrochemical element of a single cell type having a single electrochemical cell (pumping cell) with diffusion-resistance means, there is a problem that an electromotive force which is induced according to the principle of a concentration cell and which is measured as an output indicative of a partial pressure of the component to be measured or the component which reacts with the component to be measured, will have an error due to resistance polarization by a pumping current flowing through the electrochemical cell.
To solve the problem with the method using an electrochemical element of a single cell type, a method is proposed which uses an electrochemical element of a double cell type which comprises two electrochemical cells each constructed as previously described. In this method, one of the cells is adapted to serve as an electrochemical pumping cell, while the other cell is adapted to serve as an electrochemical sensing cell, so that the varying concentration of the desired component is determined by measuring an accordingly varying amount of diffusion of the component.
In the above-indicated double cell type of electrochemical element, the electrochemical pumping cell attains an electrochemical pumping action by electrode reaction due to an electric current flowing through its pair of electrodes, so as to control an atmosphere surrounding the electrode which is exposed to the gas in an external space through the diffusion-resistance means. On the other hand, the electrochemical sensing cell is adapted to detect an electromotive force which is induced, according to the principle of a concentration cell, due to a difference in partial pressure of the component to be measured or the component which reacts with the component to be measured between the atmospheres which contact the pair of electrodes of the sensing cell (one of these electrodes being exposed to the atmosphere controlled by the pumping cell indicated above). The pumping current applied to the electrochemical pumping cell is controlled so that the detected electromotive force is equal to a predetermined value. The concentration of the component in the gas to be measured is determined by the amount of controlled pumping current. Alternatively, a predetermined amount of pumping current is applied to the pumping cell to control the atmosphere surrounding one of the pumping electrodes, so that an electromotive force corresponding to the thus controlled atmosphere is measured by the electrochemical sensing cell, as an output signal of if the concentration of the component to be measured is more or less than the value which is represented by said pumping current.
3. Problem Solved by the Invention
However, the method of determining the concentration of a component in a gas by using an electrochemical element of the double cell type, suffers inconveniences as described below. That is, the atmosphere to which one of the electrodes of the pumping cell (i.e., inner pumping electrode) and one of the electrodes of the sensing cell (i.e., measuring electrode) are exposed tends to have a difference in concentration of a gas component because the two electrodes are spaced from each other. Further, there exists a difference in the concentration, due to a resistance of diffusion of porous structures such as a porous protective layer covering the electrodes, or the porous electrodes themselves. Consequently, there arises a difference in a partial pressure of the component to be measured or the component which reacts with the component to be measured between the atmosphere adjacent to the inner pumping electrode of the pumping cell and the atmosphere adjacent to the measuring electrode of the sensing cell. This difference results in errors in the measurements, and to deterioration of the pumping cell where the electrochemical element is applied to detect lean-burned exhaust gases (particularly, exhaust gases having an oxygen concentration as high as ambient air). In particular, when the electrochemical element is used to determine the concentration of oxygen in an atmosphere which, like dry air, consists of inert gases and oxygen only, the oxygen partial pressure adjacent to the inner pumping electrode is far lower than that adjacent to the measuring electrode. If, in this case, the atmosphere adjacent to the measuring electrode is controlled so as to have a low oxygen partial pressure, the atmosphere adjacent to the inner pumping electrode is extremely deficient in oxygen, whereby the zirconia solid electrolyte near the inner pumping electrode is subject to severe reduction and accordingly deteriorated.